Akka grouped stage using a List

GroupedWithList.scala

import akka.stream._
import akka.stream.stage.{GraphStage, GraphStageLogic, InHandler, OutHandler}

import scala.collection.immutable

final case class GroupedWithList[T](n: Int) extends GraphStage[FlowShape[T, immutable.Seq[T]]] {
  require(n > 0, "n must be greater than 0")

  val in = Inlet[T]("Grouped.in")
  val out = Outlet[immutable.Seq[T]]("Grouped.out")
  override val shape: FlowShape[T, immutable.Seq[T]] = FlowShape(in, out)

  override def createLogic(inheritedAttributes: Attributes): GraphStageLogic = new GraphStageLogic(shape) with InHandler with OutHandler {
    private val buf = {
      val b = List.newBuilder[T]
      b.sizeHint(n)
      b
    }
    var left = n

    override def onPush(): Unit = {
      buf += grab(in)
      left -= 1
      if (left == 0) {
        val elements = buf.result()
        buf.clear()
        left = n
        push(out, elements)
      } else {
        pull(in)
      }
    }

    override def onPull(): Unit = {
      pull(in)
    }

    override def onUpstreamFinish(): Unit = {
      // This means the buf is filled with some elements but not enough (left < n) to group together.
      // Since the upstream has finished we have to push them to downstream though.
      if (left < n) {
        val elements = buf.result()
        buf.clear()
        left = n
        push(out, elements)
      }
      completeStage()
    }

    setHandlers(in, out, this)
  }
}